In many industrial and government applications, motors are required to drive very large inertial loads in as precise a manner as is possible. For example, at Los Alamos National Laboratory, large motors drive extremely heavy xe2x80x9cneutron choppersxe2x80x9d that allow neutrons from a source to pass to a target in synchronized pulses. The neutron chopper system has a heavy spinning rotor that must be rotated in phase-locked synchronism with a reference pulse train based on an alternating current power supply that inherently has a meandering line frequency.
The actual chopper used at Los Alamos National Laboratory consists of a heavy, slotted metal cylinder that is suspended in a neutron beam and that is spun in phase-locked synchronism with 120 Hz pulses obtained from a heavy-duty electrical grid supply. The problem presented when trying to maintain precise control of the chopper is that the frequency of the electrical grid supply varies slightly from its nominal value of 60 Hz, which is, after all, only a per day average frequency. Short term deviations in the frequency of the electrical grid are approximately Gaussian in distribution, with approximately 0.015 Hz rms value, and are of exponentially diminishing spectral density with approximately 0.035 Hz FWHM (full wave half-maximum) spread. Therefore, rotational energy, in properly controlled quantities, must be continually transferred into and out of the neutron chopper in order to keep its angle of rotation in phase alignment with the zero-crossings of the power from the electrical grid.
There has been a great deal of research and development in devices intended to provide a proper level of control for this application. These devices originally were analog devices that could provide control with approximately 100 xcexcs differential with the power system zero crossings. More recently, digital circuits have been developed that provide a greater level of precision. However, the present controllers using digital signal processing are expensive and complicated.
The present invention provides an effective control system for these large loaded systems that is a hybrid between analog and digital techniques and is simpler and less expensive than those of the prior art. It uses commercially available digital signal processors along with off the shelf components to provide precision control of large loads that are to be connected to the electrical grid and synchronized with it through the motor control circuitry of the present invention.
It is therefore an object of the present invention to provide precise speed and phase control for large motor loads that must be synchronized with a a fixed or slowly varying reference signal.
It is another object of the present invention to provide speed and phase control for large motor loads that is very accurate.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a digital signal processing method for controlling the speed and phase of a motor comprises the steps of: inputting a reference signal having a frequency and relative phase indicative of a time based signal; modifying the reference signal to introduce a slew-rate limited portion of each cycle of the reference signal; inputting a feedback signal having a frequency and relative phase indicative of the operation of the motor; modifying the feedback signal to introduce a slew-rate limited portion of each cycle of the feedback signal; analyzing the modified reference signal and the feedback signal to determine the frequency of the modified reference signal and of the modified feedback signal and the relative phase between the modified reference signal and the modified feedback signal;
and outputting control signals to the motor for adjusting the speed and phase of the motor based on the frequency determination and determination of the relative phase.
In a further aspect of the present invention and in accordance with its objects and principles apparatus for controlling the speed and phase of a motor using digital signal processing comprises a reference signal having a frequency and relative phase indicative of a time-based signal for use in controlling said motor with a first slew-rate limiter receiving the reference signal for introducing a slew-rate limited portion of each cycle of the reference signal. A feedback signal having a frequency and relative phase indicative of operation of said motor with a second slew-rate limiter receiving the feedback signal for introducing a slew-rate limited portion of each cycle of the feedback signal. A digital signal processing circuit receives the slew-rate limited reference signal and the slew-rate limited feedback signal for determining the frequency of the slew-rate limited reference signal and of the slew-rate limited feedback signal, and the relative phase between the slew-rate limited reference signal and the slew-rate limited feedback signal, and for providing therefrom a control signal for controlling the frequency and phase of the motor.